High contrast is an essential factor for evaluating the picture quality of every display technologies. From this perspective, a high peak-white luminance is always required to achieve a good contrast ratio and, as a result, a good picture performance even with ambient light conditions. Otherwise, the success of a new display technology requires also a well-balanced power consumption. For every kind of active display, more peak luminance corresponds also to a higher power that flows in the electronic of the display. Therefore, if no specific management is done, the enhancement of the peak luminance for a given electronic efficacy will lead to an increase of the power consumption. So, it is common to use a power management concept to stabilize the power consumption of the display. The main idea behind every kind of power management concept associated with peak white enhancement is based on the variation of the peak luminance depending on the picture content in order to stabilize the power consumption to a specified value as illustrated on FIG. 1. In this figure, the peak luminance decreases as the picture load increases. The power consumption is kept constant.
The concept described on FIG. 1 enables to avoid any overloading of the power supply as well as a maximum contrast for a given picture. Such a concept suits very well to the human visual system, which is dazzled in case of full white picture (picture load=100%) whereas it is really sensitive to dynamic in case of dark picture (e.g. dark night with a moon). Therefore, in order to increase the impression of high contrast on dark picture, the peak luminance is set to very high values whereas it is reduced in case of energetic pictures (full white).
In the case of analog displays like Cathode Ray Tubes (CRTs), the power management is based on a so called ABM function (Average Beam-current Limiter), which is implemented by analog means, and which decreases video gain as a function of average luminance, usually measured over a RC stage. In the case of a plasma display, the luminance as well as the power consumption is directly linked to the number of sustain pulses (light pulses) per frame. As shown on FIG. 2, the number of sustain pulses for peak white decreases as the picture load, which corresponds to the Average Power Level (APL) of the picture, increases for keeping constant the power consumption.
The computation of the Average Power Level (APL) of a picture P is for example made through the following function:
      APL    ⁡          (      P      )        =            1              C        ⨯        L              ·                  ∑                  x          ,          y                    ⁢              l        ⁡                  (                      x            ,            y                    )                    where I(x,y) represents the luminance of a pixel with coordinates (x,y) in the picture P, C is the number of columns and L is the number of lines of the picture P.
Then, for every possible APL values, a maximal number of sustain pulses is fixed for the peak white pixels for keeping constant the power consumption of the PDP. Since, only an integer number of sustain pulses can be used, there is only a limited number of available APL values. In theory, the number of sustain pulses that can be displayed for the peak white pixels can be very high. Indeed, if the picture load tends to zero, the power consumption tends also to zero, and the maximal number of sustain pulses for a constant power consumption tends to infinite. However, the maximal number of sustain pulses defining the maximal peak white (peak white for a picture load of 0%) is limited by the available time in a frame for the sustaining and by the minimum duration of a sustain pulse. FIG. 3 illustrates the duration and the content of a frame comprising 12 subfields having different weights, each subfield comprising an addressing period for activating the cells of the panel and a sustaining period for illuminating the activated cells of the panel. The duration of the addressing period is identical for each subfield and the duration of the sustaining period is proportional to the weight of the subfield. When the picture load is high, the number of cells consuming energy at a given time is high; so, the duration of the sustaining period should be reduced for keeping constant the average power consumption. That is the reason why the sustaining duration for a frame is higher for a low picture load than for a high picture load.
In addition, in order to achieve a high maximal peak white, the number of subfield is kept to a minimum ensuring an acceptable grayscale portrayal (with few false contour effects), the addressing speed is increased to a maximum keeping an acceptable panel behavior (response fidelity) and the sustain pulse duration is kept to a minimum but having an acceptable efficacy.
But, at this stage, PDP makers are faced with another problem called load effect explained below. As previously mentioned, a high peak white requires to be able to shorten the duration of a sustain pulse. However, this increase of the sustain frequency has a strong drawback: it increases load effect, especially, when the xenon percentage in the gas of the PDP cells is high. This effect is illustrated by FIG. 4. It represents a white cross on a black background. Losses due to line capacity effect occur and have a strong influence on the panel luminance for a high sustain frequency. The white horizontal lines of the cross are less luminous in a high sustain frequency mode (right part of FIG. 4) than in a low sustain frequency mode (left part). This example shows a line load effect.
The line load effect itself represents a dependence of subfield luminance towards its horizontal distribution. In that case, it does not matter to know the load of the subfield but rather to know the differences of load between two consecutive lines for the same subfield.
When the subfield distribution is “geometrical”, e.g. for displaying artificial geometrical patterns, the line load effect is much more critical than for video pictures which suffer mainly from a global load effect.
Generally the load effect is not only limited to the line load but also to a global load of the subfield in a frame. Indeed, if a subfield is globally more used than another one on the whole screen, it will have less luminance per sustain pulse due to this load effect (the losses occur in the screen and in the electronic circuitry).
Therefore, on the one hand, a high number of sustain pulses and a high sustain frequency are required for peak white modes and, on the other hand, the panel will lose its homogeneity in case of peak white modes. This can have dramatic effects on natural scene as shown in FIG. 5.
The load effect has an impact on the grayscale portrayal under the form of a kind of solarization effect which looks like a lack of gray levels. In that case, the right picture seems to be coded with fewer bits than the left one. This is due to the fact that some subfields are suddenly less luminous than they should be. In that case, if we consider two video levels that should have similar luminance, and if one of them is using such a subfield, its global luminance will be too low compared to the other video level introducing a disturbing effect.
An object of the method of the invention is to reduce the line load effect that is directly linked to the capacity of a line and not the global load effect that can be compensated by other methods. The method of the invention can be used independently to those methods when a PC mode is selected or in addition to one of them since they are compatible.
Globally, the invention is based on a profile analysis of the line load for each subfield to determine if this subfield is more or less critical to line load effect. If such a subfield is detected, its sustain frequency is reduced to minimize the load effect.